Wireless stations in wireless local area systems are powered from a battery. Different kinds of wireless stations are known, and they may be mobile. Examples of wireless stations are Laptops, Palmtops, various kind of handhelds, but also other wireless stations are known, and still other types are expected to be introduced in the future. A general requirement is that the batteries used for power supply should be small. On the other hand, a high output power on the air interface is required in order to ensure a good contact with access points, base stations etc, or with other wireless stations, e.g. for systems operating without a real access point, where instead each wireless station may act as a temporary access point. See commonly assigned Swedish application No. 0103394-3, entitled “Wireless Networks and a Method of Reducing Signalling in a Network”, Lindskog, filed on Oct., 12, 2001 (now abandoned), and the content of which herewith is incorporated herein by reference. This may be in conflict with the requirement that the batteries should be as small as possible. Existing WLAN systems on 2.4 GHz frequency band, e.g. IEEE 802.11b, have a fairly low power consumption and subsequently acceptable requirements upon existing interface standards, e.g. USB, PC-Card etc. In order to save battery power, the active periods for the wireless station have been reduced as much as possible, e.g. through the introduction of so called sleep modes which may be implemented in different manners, i.e. entry of a sleep mode after detecting inactivity for a given time period, a wireless station enters a sleep mode only transmitting page signals, by paging until a response is received, etc. Thermal problems can be solved by making pauses, cooling down, etc.
With recently developed systems or with developing systems the problems associated with available power contra required output power have become even more apparent in that increased data throughput for WLAN products, e.g. IEEE 802.11a, requires a higher power consumption. In such systems a wireless station often comprises network interface means in the form of cards, e.g. so called Network Interface Cards (NIC), which are provided with energy holding means or auxiliary power supply means such as charge reservoirs, e.g. capacitor banks, built at the transmitter of the NIC.
In addition thereto standards often require that long frames can be sent, for example at least 1500 byte frames. Transmission of such a long frame requires a lot of energy. All this energy may not be taken directly from the energy holding means, e.g. the battery, of the wireless station. Therefore the above mentioned charge reservoirs or capacitor banks may be used. It is however both expensive and space demanding to build large charge reservoirs at the NIC transmitter. Also the network interface between wireless station and NIC is a limiting factor on long transmit times for long frames and constitutes a problem for small devices or wireless stations with a limited battery power, since the energy consumption during the time it takes to transmit may be higher than what can be charged from the interface towards a mobile host. It is a problem that sufficient energy/power can not be provided even if a so called charge reservoir is used in addition to the battery. IEEE 802.11 is one example a WLAN standard requiring that at least 1500 byte frames can be sent (particularly 2346 byte frames).
IEEE has recently added a 5 GHz high-rate physical layer (IEEE 802.11a) to the wireless LAN (WLAN) standard 802.11. This new physical layer uses Orthogonal Frequency Division Multiplexing (OFDM) as a modulation technique. Previously used physical layers (IEEE 802.11 and 802.11b) operate at a lower rate, in the frequency band of 2.4 GHz and use Frequency Hopping (FH) or Direct Sequence Spread Spectrum (DSSS) modulation.
OFDM is a modulation technique which requires highly linear amplifiers which means that it is a very power inefficient technique. For comparison, FH and DSSS are much more power efficient. The power output from the antenna of a WLAN NIC has to be high enough to support a reasonable coverage area. The power consumption of a WLAN card, on the other hand, has to be low enough in order do not too quickly drain the battery of the wireless station. The maximum peak current from a wireless station is also limited, particularly for handheld devices. The maximum peak current may also be limited due to the interface, e.g. Card Bus or USB, towards the WLAN NIC or due to current limitations from the mobile host, e.g. PC.
Using the 6 Mbps Physical mode (PHY), which is the slowest PHY mode of IEEE 802.11a, the longest possible MAC frame will take 3.1 ms to transmit, including preamble. This is a very long transmit time compared to the maximum continuous transmit time of for example HIPERLAN type 2, which is a comparable WLAN standard using OFDM in the 5 GHz frequency band.
IEEE 802.11 uses so called MPDU, (Medium access control Protocol Data Unit) frames to transmit data over the wireless medium, (WM). The maximum size of a frame, MSDU, header and FCS (Frame Check Sequence) is 2346 octets, and, as referred to above, the long transmit times of frames will constitute a serious problem for small wireless stations. Implementation of various kinds of sleep functionalities will not solve the problem, since the long frames still have to be sent according to the standard. If instead the transmission range is reduced, this will result in other problems, e.g. in that more access points will be needed.
Thus, there is no satisfactory solution known to the problem of keeping down the power requirements while still being able to meet the requirements as to transmission of long frames and coverage range, or the problem of the output power requirements contra available power supply in a wireless station.